To increase the speed with which an integrated circuit can process information, it is necessary to reduce the distance that the electrical signals have to travel. While this can be accomplished by reducing the size of the circuit components, it can also be accomplished by reducing the electrical resistance in the circuit, which reduces power dissipation and permits closer packing of electrical components. In a superconducting circuit, the electrical resistance is reduced to zero by cooling the circuit below the temperature at which its components become superconducting. While commercial superconducting electronics with medium or large scale integration have not yet been built, some research has been done on designing the components.
The state of the art design for a Josephson tunnel junction for superconducting electronics is a trilayer, on a substrate, formed of a layer of superconducting niobium nitride (NbN), an insulating layer of magnesium oxide (MgO), and a second layer of niobium nitride on top. While this structure is operable, it is not optimal because the lattice spacing of magnesium oxide does not match the lattice spacing of niobium nitride, although they do not have the same structure and crystal orientation. Niobium nitride has a B1 (sodium chloride) structure with a lattice constant, (a.sub.o) of 4.38 angstroms; magnesium oxide and calcium oxide have the same B1 structure, but magnesium oxide has a lattice constant of 4.21 angstroms. Because of this mismatch, the initial layer of the top niobium nitride will be partly disordered, the superconducting energy gap reduced, and the operating temperature of the junction reduced.